Polymethylmethacrylate-based thermoplastic polymers that are methacrylic resins have characteristics exhibiting excellent transparency that is high total light transmittance in the visible light region and surface hardness, and are therefore used in various fields. However, the methacrylic resins are insufficient in mechanical properties, in particular, in impact resistance and toughness for applying to some purposes and have been required to be improved.
In order to improve mechanical properties, usually, core-shell particles composed of a rubber layer and a methacrylic resin layer synthesized by emulsion polymerization are blended with a polymethylmethacrylate-based thermoplastic polymer (methacrylic resin). However, though a molded article made of a composition prepared by this method has an improved impact resistance, the toughness is not sufficiently improved. In addition, the blending of the rubber component causes decreases in surface hardness, rigidity, and heat resistance. Furthermore, when, for example, tension stress or bending stress is applied, whitening occurs in the area the stress is concentrated in some cases. Whitening may also occur when the molded article is applied with impact or left under high temperature and humidity conditions for a long time. The whitening leads to loss of the transparency and a tendency of deterioration of the design characteristics and the quality of the molded article.
As another method for improving the toughness of a methacrylic resin, proposed is a method where methyl methacrylate is copolymerized with another monomer that reduces the glass transition temperature. However, this method has problems in that the rigidity and the heat resistance are considerably decreased.
A methacrylic resin composition prepared by blending a methacrylic resin with another polymer is also proposed.
The polymer blended with the methacrylic resin is, for example, a styrene-acrylonitrile copolymer having a specific composition, polyvinyl chloride, polyvinylidene fluoride or the like. However, the toughness cannot be sufficiently improved by blending these polymers.
Blending of polyethylene oxide is also proposed. The polyethylene oxide is excellent in compatibility with polymethylmethacrylate, and, therefore, improvement of toughness can be expected. However, since the glass transition temperature is low, decreases in the rigidity and the heat resistance of the composition cannot be avoided.
Polycarbonate is a polymer that can be expected to improve balance among toughness, heat resistance and transparency. It is reported that a transparent composition composed of bisphenol A polycarbonate and polymethylmethacrylate is obtained by, for example, dissolving polymethylmethacrylate and polycarbonate in tetrahydrofuran, adding the solution to heptane for precipitation, and heating the precipitate at a temperature higher than the glass transition temperatures of the polymethylmethacrylate and the polycarbonate. However, molded articles made of this composition have low surface hardness. Furthermore, since a solvent is used for preparing the composition, great deal of energy is necessary for removing the solvent, resulting in low productivity. Melt-kneading of polycarbonate and polymethylmethacrylate is also reported. However, articles molded from the melt-kneaded composition are opaque and pearlescent because of phase separation of the polycarbonate and the polymethylmethacrylate (Non-Patent Document 1).
Polyvinyl butyral is a candidate polymer potentially miscible with polymethylmethacrylate.
Non-Patent Document 2 discloses that since a methyl methacrylate resin and polyvinyl butyral are weakly miscible, products obtained by mixing them usually have a two-phase structure as a result of phase separation, however, that by using a methyl methacrylate resin having a low molecular weight in the mixing, both are possibly miscible to form a single phase. FIG. 5 in Non-Patent Document 2 shows an optical microscope image of a film obtained by dissolving a blend of 50 parts by mass of methyl methacrylate resin and 50 parts by mass of polyvinyl butyral containing various amounts of vinyl alcohol units in a solvent, and carrying out cast molding. This film has a phase separation structure in which the methyl methacrylate resin composes various sizes of dispersed phases.
Non-Patent Document 3 discloses blends obtained by melt-kneading polyvinyl butyral and polymethylmethacrylate having a weight-average molecular weight of 120,000 in various proportions. According to Non-Patent Document 3, blends containing larger amounts of polyvinyl butyral show greater elongation at breaking in a tension test, yield behavior, and improved toughness. However, the blends containing larger amounts of polyvinyl butyral described in Non-Patent Document 3 had insufficient mechanical properties. In contrast, in blends containing polyvinyl butyral in the amount of less than 50 mass %, toughness was hardly improved, and mechanical properties were insufficient.
Furthermore, Patent Document 1 discloses a resin composition composed of a plasticized polyvinyl acetal resin and a block copolymer containing a methacrylic copolymer block and an acrylic polymer block. According to Patent Document 1, this resin composition is used for bonding two glass plates and is suppressed in a whitening phenomenon due to contact with air. However, the resin composition has very low surface hardness because of a large amount of plasticizer contained therein, and the mechanical properties are also insufficient.